The present invention relates to ultraviolet rays (UV)-curable resin compositions, which provide cured products having high elastic moduli, high elongations, low water-absorption and low viscosities. These UV-curable resin compositions are utilized as coating material and are especially effective as secondary coating material for optical fibers.
Optical fibers are in general subjected to coating with resin along with the drawing operation, since possible minute defects on the surface of fibers may give rise to a drop in the intensity of light. The coating generally comprises two layers, consisting of a primary coating material having a lower modulus for direct protection of the surface of fibers and a secondary coating material having a higher modulus to render strengths to fibers. Furthermore, UV-curable resins have increasingly been utilized in recent years as coating material for increasing drawing speed to improve productivity.
Among other properties required for the secondary coating materials, appropriate elastic modulus and elongation are required in order to maintain mechanical strengths, thereby preventing increase in optical transmission loss due to microbending. In addition, from the viewpoint of preventing humidity and water, they are required to have a low water-absorption. Furthermore, to increase drawing speed at processing operation, thereby improving productivity, a high curing rate is required.
Hitherto, as the secondary coating material for optical fiber, such materials which have elastic modulus of more than 10 kg/mm.sup.2 and elongations of more than 10% have been used to maintain mechanical strengths and to prevent increase in optical transmission loss due to microbending. Further, to maintain water-resisting and humidity-resisting properties, materials having a water-absorption of less than 4% are used. Furthermore, for the purpose of improving productivity by drawing optical fibers at high speeds, materials which have viscosities less than 15000 cps at normal temperature (25.degree. C.) are preferably used so far. However, these coating materials have some defects. For example, some of them have higher water-absorptions, though elastic moduli thereof are satisfactory, and the other ones have satisfactory elastic moduli and elongations, but viscosities thereof are high and furthermore curing rates are slow. There have been no materials which satisfy all of these characteristic properties properly.
Furthermore, the secondary coating layer becomes thinner as the diameters of optical fibers are reduced step by step, and therefore coating materials which have higher elastic moduli and elongations, sufficient toughness and well-balanced properties are required.